In providing lubricants for internal combustion engines, such as those used in passenger cars and trucks, it is desirable to provide antioxidant additives that improve the useful life of the lubricants and reduce engine corrosion.
Antioxidants in the form of hindered, sulfur-bridged phenols having a branched alkyl group on the alpha carbon atom and made by reacting the phenol with sulfur dichloride in a solvent with recovery of a crystalline product from the reaction mixture are disclosed in U.S. Pat. No. 3,250,712.
The preparation of a crystalline 4,4-thiobis-(2,6-di-t-butylphenol) product by reacting the phenol with a sulfur halide in a solvent such as acetonitrile, carbon disulfide or carbon tetrachloride with or without a catalyst followed by treating the reaction mixture with alkali hydroxide in alcohol is disclosed in U.S. Pat. No. 3,678,115.
The preparation of a liquid lubricant oil additive mixture of 45-75 wt % ortho-alkylphenol and certain amounts of mono-, di-, tri- and tetrasulfides of the phenol by the reaction of an excess of the phenol with sulfur using an organic amine catalyst is disclosed in U.S. Pat. No. 3,929,654. It is also reported in this patent that sulfurized alkylphenols prepared by reacting an alkylphenol with sulfur mono- or dichloride tend to cause copper corrosion probably due to the presence of corrosive sulfur species such as sulphochlorinated alkylphenol.
U.S. Pat. No. 4,946,610 discloses a liquid, sulfur-bridged, hindered phenol antioxidant composition that is prepared by reacting a mixture of hindered phenols with a sulfur chloride in the presence of a polar modifier. The composition is an effective antioxidant in lubricating oils without causing excessive copper corrosion.
Co-pending application, Ser. No. 08/657,141, filed Jun. 3, 1996, relates to the preparation of sulfurized t-butylphenol oligomers using solvents analogous and homologous to 1-propanol, e.g., ethanol, 2-propanol, 1-butanol, and isobutyl alcohol. The products taught in Ser. No. 08/657,141 have less oligomeric products than the products of the present invention.
One of the objects of this invention is the preparation of an oil soluble, chlorine-free, sulfurized hindered phenolic antioxidant that is compatible with elastomer seals, does not cause excessive copper corrosion and is a highly effective antioxidant in low phosphorus (&lt;1000 ppm phosphorus) lubricating oils used primarily in passenger cars and trucks.